Method for carrying out polycondensation reactions

ABSTRACT

The invention relates to a method for carrying out polycondensation reactions, according to which the polycondensation of a monomeric starting material is carried out with external supply of heat in a reactor combination which has at least two stages and is composed of a pre-reactor and a high-viscosity reactor, where the low-molecular-weight elimination products produced are removed by evaporation. In the pre-reactor, the reaction product is concentrated to give a high-viscosity preliminary product. The high-viscosity preliminary product is then fed to the high-viscosity reactor, in which it reacts to completion with simultaneous introduction of thermal and mechanical energy and with a residence time of from 20 s to 60 min to give a polycondensation product.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A polycondensation reaction is a chemical reaction in which amacromolecule is built up stepwise (Emons, H. H.; Fedtke, M.; Hellmond,P.; Landschulz, G.; Pöschl, R.; Pritzkow, W.; Rätzsch, M.; Zimmermann,G.; Lehrbuch der Technischen Chemie [Textbook of Industrial Chemistry];VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1984). Each stepin the condensation produces a reaction product which is in equilibriumwith other reaction constituents. The reaction is therefore anequilibrium reaction. In each case the reaction takes place between twodifferent functional groups of the starting materials (monomers), and ateach stage of the reaction a low-molecular-weight substance (e.g. water,hydrogen halides, alcohols, etc.) is eliminated, with simultaneouslengthening of a polymer chain by one monomeric building block. Productsof the reaction are therefore the macromolecule and the correspondinglow-molecular-weight elimination products, in equilibrium with thestarting materials.

If a high conversion is desired from a polycondensation reaction thelow-molecular-weight elimination products must be removed from theequilibrium in order to shift the reaction equilibrium toward theproducts. If the monomers are dissolved in a solvent at the beginning ofthe reaction, there may also be a need for the solvent likewise to beremoved from the reaction mixture. It is possible here to use thelow-molecular-weight elimination products as a solvent.

When the low-molecular-weight elimination products and the solvent, ifused, are removed the viscosity of the reaction mixture can change froma low-viscosity solution (e.g. similar to water) at the beginning of thereaction to give a high-viscosity polymer melt or polymer solution atthe end of the reaction. Indeed, it is frequently necessary to removethe low-molecular-weight elimination products and the solvent, if used,through as far as a dry solid if the desired conversion in the reactionis to be achieved.

2. Description of the Related Art

The familiar method for removing the low-molecular-weight eliminationproducts and the solvent, if used, is distillation. This means thatwhile the polycondensation reaction progresses the low-molecular-weightelimination products and, respectively, the solvent, if used, areremoved by evaporation, either simultaneously or in stages (alternatingreaction and distillation).

Chemical reactors for carrying out polycondensation reactions thereforehave two tasks. They must be able to mix and transport the reactionmixture efficiently at low, and also at high, viscosities (whereappropriate through as far as dry solids) and at the same time allowremoval by evaporation of the low-molecular-weight elimination productsand/or also the solvent from the reaction mixture.

The following reactors are used in prior art methods forpolycondensation reactions:

Screw Reactors

High-capacity screw reactors of ZDS-R type have been used by OCKER,Werner and Pfleiderer, Stuttgart since as early as 1962 forpolycondensing polyesters. The devices are used at low rotation ratesand with long residence times (from 1 to 2.5 hours). The process isdescribed in Herrmann: Schneckenmaschinen in der Verfahrenstechnik[Screw Devices in Processing], Springer Verlag 1972. A disadvantage ofthese devices is their low mixing efficacy, due to the low rotationrates.

Disk Reactors (Zimmer, Frankfurt Am Main)

This type of reactor is a cost-effective alternative to the screwreactor and is nowadays used worldwide for polyester production. Theprinciple on which the reactor is based is that of slowly rotating diskswhich produce melt films and thin layers which form a large surface forthe transfer of material. In the usual embodiment, the disk reactors arenot self-cleaning. One version of the reactor which has been equippedwith strippers to improve self-cleaning is still being tested on a pilotscale. Like the screw reactor, the reactor can be used over a wideviscosity range. However, its functioning requires that the melt becapable of forming a reservoir. Conversion to a non-flowable paste or tothe solid is not possible.

Twin-screw Extruders

Recently, corotating twin-screw extruders with low capacity and highrotation rates have been used for polycondensation. Example: ZSK typefrom Werner and Pfleiderer, Stuttgart or ZE type from Berstorff,Hanover. GREVENSTEIN, A.: Reaktive Extrusion und Aufbereitung [ReactionExtrusion and Product Treatment], Carl Hanser Verlag 1996, givespolyethylene terephthalate (PET), polybutylene terephthalate (PBT),copolyesters, polyimide (PI) and polyetherimide (PEI) as applications.The efficacy of mixing is good due to the high rotation rates. At thesame time there is high shear and dissipation of energy, and this canhave an adverse effect on product quality of sensitive polymers.However, the low capacity of the reactor means that this type is ofinterest only for processes which require a low residence time(generally <1 minute). For this reason industrial use is mostlyrestricted to postcondensation.

Grid-cage Reactors (e.g. Werner and Pfleiderer)

This type of reactor supplies a large reaction capacity and thereforelong residence times, and it is used on an industrial scale forpolycondensation reactions. However, compared with the other types ithas restrictions with regard to the maximum polymer viscosity which canbe processed.

High-capacity Kneading Reactors (e.g. List)

This type approaches the twin-screw extruder in its mixing efficiencyand kneading efficiency. However, large capacity means that it is alsopossible to realize high residence times. Unlike reactor types 1 to 3,however, the axial back mixing and transporting action of these reactorsis highly viscosity-dependent, i.e. at low to moderate viscosityback-mixing is at a high level and transporting action is poor. Thistype of reactor is therefore of relatively little interest industriallyfor use with low-viscosity media.

BRIEF SUMMARY OF THE INVENTION

It has been found that significantly improved product quality can beachieved in polycondensation reactions if the polycondensation of amonomeric starting material is carried out with external supply of heatin a reactor combination which has at least two stages and is composedof a pre-reactor and a high-viscosity reactor, where thelow-molecular-weight elimination products produced are removed byevaporation and the reaction product in the pre-reactor becomesconcentrated to give a high-viscosity preliminary product. The viscosityof the highly viscous preliminary product should be greater than 200mPas, preferably greater than 500 mPas. The high-viscosity preliminaryproduct is then fed to the high-viscosity reactor, in which it reacts tocompletion with simultaneous introduction of thermal and mechanicalenergy and with a residence time of from 20 s to 60 min to give apolycondensation product. The pre-reactor is an apparatus which ensuresefficient and intensive heat exchange. Any type of apparatus suitablefor heat exchange and having an operating capacity sufficient forcarrying out the chemical reaction can be used for this (e.g. atube-bundle heat exchanger, a falling-film evaporator, a plate heatexchanger, a temperature-controlled static-mixer (TSM) reactor, a mixingvessel with specific stirrer geometry for viscous products, etc.). Thepre-reactor may also be a combination of the heat exchangers.

The high-viscosity reactor's heat supply and supply of mechanical energyis sufficient to mix the reaction mixture and set the same in motion,and also to renew the surface of the same, and its reactor capacity issufficient to ensure that the residence time is achieved, and it alsohas the ability to process relatively highly viscous materials todryness. Particular preference is given to the break-up of the resultantsolid in this process to give a large number of small particles. Thisbreak-up considerably improves the evaporation and, respectively,removal of the substance eliminated during the condensation, andconsiderably reduces the diffusion path lengths for the substanceseliminated. Alongside the more effective removal of the substanceseliminated, the large surface of the solid particles markedly improvesheat transfer, leading to fully reacted product. The resultant producthas a markedly smaller amount of residual monomers and markedly bettercharacteristics in chemical analysis and in use.

Preference is given to the use of a helical-tube evaporator, or anotherheat exchanger in combination with a helical-tube reactor, aspre-reactor, and a high-capacity kneading reactor in which, usingrotating kneading elements and/or shearing elements, thepolycondensation product is agitated and comminuted, as high-viscosityreactor.

The starting fluid is, for example, firstly pumped through a heatexchanger with single- or multiphase operation and enters a spiral tubevia a pressure-release valve, with some evaporation. As previouslystated by CASPER in CIT 42 (1970), No. 6, pp. 349 et seq., turbulentannular flow of the liquid develops in the helical tube and ensures goodheat and material transfer, even when the viscosity rises through thereaction. The product, to some extent concentrated via evaporation andprecondensed in the helical tube, is fed to the high-capacity kneadingreactor. The polycondensation is progressed in the high-capacitykneading reactor, with constant and thorough mixing. During this theviscosity rises further. In specific cases the material becomes a solidwhich is no longer flowable. Any commercially available kneading reactormay be used for the novel process, as long as it is capable of achievingthe abovementioned objectives. Our example uses a CRP type reactor fromList AG, Arisdorf, Switzerland. Equipment with reinforced rotors isparticularly preferred.

The evaporated low-molecular-weight elimination products and, whereappropriate, the solvent may either be drawn off in the pre-reactor,downstream of the pre-reactor, or in the high-viscosity reactor, ordischarged with the product from the reactor combination according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

The advantages of the reactor combination according to the invention aredescribed below using the preparation of the sodium salt of polyasparticacid (Na-PAA) and, respectively, the polysuccinimide (PSI) intermediate.

To prepare polysuccinimide, an aqueous solution of an ammonium salt ofmaleic acid and/or an aqueous solution of an ammonium salt of maleicacid with low-molecular-weight adducts of an ammonium salt of maleicacid is firstly prepared from the starting materials maleic anhydride(MA) and ammonia (NH₃), batchwise or continuously, and then polymerizedcontinuously in the reactor combination according to the invention togive polysuccinimide. During this, polycondensation reactions andintramolecular cyclocondensation reactions, and others, will take place.To achieve a high degree of conversion (or a high molecular weight) inthese condensation reactions, both the solvent water and the water ofreaction eliminated during the condensation must as far as possible beremoved from the reaction mixture.

The monomeric starting material may preferably be obtained by reacting1,4-butanedicarboxylic acid or 1,4-butenedicarboxylic acid or aderivative thereof with ammonia or with a compound supplying ammonia,e.g. urea, ammonium salts of carbonic acid, ammonium salts of phosphoricacid or formamide.

Other starting materials which may be used in the novel method insteadof maleic anhydride are maleic acid, fumaric acid, malic acid, asparticacid and asparagine, and also mixtures of these. Other cocondensablemonomers may also be added to the reaction mixture in the reactorcombination according to the invention. Examples of cocondensablecompounds which may be used are fatty acids, polybasic carboxylic acids,their anhydrides and amides, polybasic hydroxycarboxylic acids, theiranhydrides and amides, polyhydroxycarboxylic acids, aminocarboxylicacids, sugar carboxylic acids, alcohols, polyols, amines, polyamines,aminoalcohols, amino sugars, carbohydrates, ethylenically unsaturatedmono- and polycarboxylic acids, protein hydrolysates, e.g. maize proteinhydrolysate and soy protein hydrolysate, and aminosulfonic acids. Topromote the condensation, condensation auxiliaries may also be added tothe reaction mixtures. Examples of these are phosphoric acid,polyphosphoric acid, phosphorous acid, phosphonic acid and acid salts,such as sodium hydrogensulfate, potassium hydrogensulfate and ammoniumhydrogensulfate. In a preferred embodiment, these condensationauxiliaries are added to the reaction mixture in the final reactionstage in the high-viscosity reactor.

There is a direct correlation between high conversion and good productquality, i.e. good usage properties acceptable to customers (e.g.: ZnOdispersion test, NACE test).

In one embodiment of the invention, the polymers obtained in thehigh-viscosity reactor in the second reaction stage can then besubjected to solvolysis, preferably hydrolysis. The resultant polymerpreferably has essentially recurring aspartic acid units.

These polymers are used with advantage in aqueous or nonaqueous systemsfor dispersing inorganic or organic particles, and in particular forinhibiting and dispersing precipitates in water treatment.

EXAMPLES

As a basis for comparison, experiments were firstly carried outaccording to the prior art with a single reactor.

a) Helical-tube Reactor

The reaction mixture must be liquid for processing in a helical-tubereactor. It may be concentrated by evaporation during this to give aviscous melt. When using a helical-tube reactor or pre-reactor, productquality is markedly poorer than with the reactor combination accordingto the invention (see Examples, M_(W), ZnO test, NACE test, testdescription see below). The pre-reactors, such as a helical-tubereactor, are simple and low-cost apparatuses with high throughputs.

b) High-viscosity Reactor

A high-viscosity reactor from List was the sole reactor used. Thereaction mixture cannot be concentrated by evaporation to dryness at thethroughputs required. A List reactor is not suitable for processing lowviscosities. The low-viscosity starting material “flows rapidly throughthe reactor”. The apparatus has high equipment costs per unit. Theentire procedure for concentrating the low-viscosity aqueous solution byevaporation to give the solid via the high-viscosity melt/solution iscarried out in an apparatus specifically for processing relativelyhigh-viscosity substances. Product quality is markedly poorer than withthe reactor combination according to the invention (see Examples, M_(W),ZnO test, NACE test, test description see below).

c) Novel Process Using a Reactor Combination

The pre-reactor consisted of a helical-tube reactor and thehigh-viscosity reactor consisted of a List reactor. The novel processwith the reactor combination helical-tube/List reactor is the bestprocess when compared with the processes using only one reactor. In thehelical-tube reactor the low-viscosity starting-material solution iscondensed to give a relatively high-viscosity melt/solution. Therelatively high-viscosity melt/solution, which should have a viscosityof more than 200 mPas, preferably more than 500 mPas, is then fed to thedownstream List reactor, in such a way that the advantages of thereactor can be fully utilized. The significantly larger reactor capacitypermits a relatively high residence time and thus a lower reactiontemperature. This results in an efficient and more gentle method ofproduction, and this is seen in the product quality, which is the bestachieved (see Examples, M_(W), ZnO test, NACE test, test description seebelow).

The reaction and the concentration by evaporation in the pre-reactor iscarried out with a residence time of from 0.5 to 300 minutes, preferablyfrom 1 to 20 minutes and particularly preferably from 2 to 10 minutes,at temperatures above 100° C., preferably from 100 to 250° C. andparticularly preferably from 110 to 210° C., and at pressures of from0.01 to 100 bar, preferably from 0.1 to 25 bar and particularlypreferably from 1 to 10 bar. In the high-viscosity reactor, temperaturesare set at from 100 to 350° C., preferably from 120 to 250° C. andparticularly preferably from 140 to 220° C., and pressures at from 0.01to 10 bar, preferably from 0.1 to 3 bar and particularly preferably from0.5 to 2 bar, with residence times of preferably from 20 seconds to 60minutes and particularly preferably from 1 minute to 30 minutes.

The starting-material solution of an ammonium salt of maleic acid may beprepared batchwise or else continuously from water, maleic acid andammonia, and fed to the reactor combination. The molar ratio of nitrogenin the ammonia to maleic acid here is from 0.1 to 25, preferably from0.5 to 8 and particularly preferably from 0.9 to 4. The proportion ofwater in the solution is from 20 to 90% by weight, preferably from 20 to60% by weight and particularly preferably from 25 to 40% by weight. Ifthe starting-material solution is prepared batchwise, precondensation inwhich an average of up to 2 molecules combine may occur in the storagetank.

The following usage tests and evaluation methods were used to comparethe polycondensation products produced:

Determination of threshold effect (inhibition of calcium carbonateprecipitation via substoichiometric addition of inhibitor) by a modifiedNACE¹) method:

¹⁾ NACE: National Association of Corrosion engineers

Solutions Required:

1. 12.15 g of calcium chloride dihydrate analytical grade 68 g ofmagnesium chloride hexahydrate analytical grade made up to 1000 ml ofsolution with distilled CO₂-free water.

2. 7.36 g of sodium hydrogen carbonate analytical grade made up to 1000ml of solution with distilled CO₂-free water.

3. 1000 mg of the inhibitor substance to be studied made up to 1000 mlof solution with distilled CO₂-free water.

Solutions 1. and 2. are to be filtered through a 0.45 μm membrane filterbefore use and saturated with carbon dioxide. Inhibitor solutionscorresponding to the test concentration desired are precharged to 250 mlnarrow-neck glass flasks:

ppm of inhibitor μL of inhibitor solution 1 200 2 400 3 600 5 1000  10 2000 

Using a 100 ml dispensette, 100 ml of each of solutions 1. and 2. areinjected onto the precharged solutions. The flasks are then immediatelysealed, shaken once by hand, and placed in a waterbath heated to 70° C.The samples are stored for 16 hours at this temperature. As a comparisona sample is run alongside without addition of inhibitor. (To determinethe initial value, calcium content is determined by titrationimmediately after mixing solutions 1. and 2.)

After this time the samples are removed simultaneously from thewaterbath and slowly cooled to a temperature of 30° C. A 5 ml sample ofeach is then filtered through a 0.45 μm membrane filter into about 100ml of distilled water and stabilized by acidifying with 0.5 ml ofconcentrated hydrochloric acid. The determination of calcium contentthen required is carried out by titration with an indicator.

Percentage inhibition is calculated as follows:${\frac{a - b}{c - b} \cdot 100} = {\% \quad {inhibition}}$

a: Amount of calcium found in the sample

b: Amount of calcium in the blind sample (after heat-treatment)

c: Amount of calcium in the blind sample (prior to heat-treatment)

Determination of Solid-dispersion Performance Using Zinc Oxide

1 g of the dispersing agent to be studied is dissolved in 50 ml ofdistilled water. The pH of the sample should be 10. The sample preparedin this way is transferred to a 100 ml measuring cylinder and the volumemade up with distilled water (stock solution).

10.0 g of ZnO analytical grade (Merck) is precharged to a 250 ml mixingcylinder and slurried with from 140 to 170 ml of water. The followingamounts of dispersing agents are used for this.

 50 ppm  1 ml stock solution  100 ppm  2 ml stock solution  250 ppm  5ml stock solution  500 ppm 10 ml stock solution 1000 ppm 20 ml stocksolution 1500 ppm 30 ml stock solution

The mixture is predispersed using a disperser (e.g. Ultraturrax stirrer)for 30 sec and then made up to 200 ml. The final sample suspension isshaken three times by hand and stored for 3 hours at room temperature.Using an ordinary 5 ml pipette, an aliquot is then removed at the 150 mlmark and transferred to a 50 ml measuring flask to which 10 ml of 1 Nhydrochloric acid and about 20 ml of water have been precharged. Aftermaking up the volume in the measuring cylinder, an aliquot of 10 ml isremoved and titrated at pH 11 with EDTA solution, with an indicator.

Evaluation $\frac{V*t*81.37*5}{0.025*100} = {\% \quad {ZnO}}$

V=volume of EDTA solution

t=titer of EDTA solution

81.37=molar mass of ZnO

5=50/10 derived from the material taken from the HCl measuring flask

0.025=5/200 derived from the material taken from the cylinder

100=%

Example 1

The conduct of the example described below corresponded to the generalmethod description c) using a reactor combination.

Preparation of Solution of an NH₄ Salt of Maleic Acid

51.7 kg of H₂O are precharged at a temperature of 60° C. to a 250 lvessel and 75 kg of solid maleic anhydride are added, giving a maleicacid solution. 16.9 kg of ammonia (gaseous) are then metered in, withcooling, at from 90 to 100° C. The resultant solution of an NH₄ salt ofmaleic acid is temperature-controlled at from 100 to 105° C. and pumpedat 41 kg per hour into a polycondensation plant.

Preparation of Polysuccinimide

The condensation plant is composed of a pre-heater of length 8.4 m(internal diameter 10 mm) in which the solution is heated to 192° C. ata pressure of 10 bar. From the pre-heater, the solution passes via anorifice into a helical-tube evaporator of length 15 m (internal diameter15 mm) in which the reaction solution reaches a temperature of 193° C.and a pressure of 2.9 bar downstream of the orifice. The reactionmixture at 195° C. is passed via a pipeline of length 6 m into a List(CRP 12 Konti) kneading apparatus. In the List reactor the reactionmixture is concentrated by evaporation to dryness at temperatures offrom 190 to 195° C. and rotation rates of 31/min, and during this ispolymerized to completion. An amount of about 21 kg per hour of theresultant granular polysuccinimide is obtained. Its hydrolysis number is10.61 mmol of NaOH/g of PSI.

Preparation of Solution of an Na Salt of Polyaspartic Acid 2100 g ofwater and 360 g=9 mol of NaOH are precharged, and 1000 g ofpolysuccinimide are added little by little at 20° C., with stirring.During this the temperature rises to 60° C. through exothermicity andthe PSI dissolves. A further 64.4 g=1.61 mol of NaOH are added, thetemperature is increased to 100-110° C. and, with addition of 2×700 g ofwater, 3×700 g of ammonia-water are distilled off. After adding 175.6 gof water, 3000 g of a 43.7% strength by weight solution of an Na salt ofpolyaspartic acid are obtained.

Molecular weight distribution ZnO dispersion test NACE test by GPCM_(w)10-300 mg 3 ppm/10 ppm [g/mol] [% of theory] [% of theory] 2350 8078/100

Example 2

The example described below was carried out using a single reactor as inthe general method description a) (prior art).

Preparation of Solution of an NH₄ Salt of Maleic Acid

An amount of 40 kg/h of solution of an NH₄ salt of maleic acid,temperature-controlled at from 100 to 105° C. and prepared as in Example1, is pumped into a plant for polycondensation.

Preparation of Solution of an Na/NH₄ Salt of Polyaspartic Acid

The condensation plant is composed of a pre-heater of length 8.4 m(internal diameter 10 mm) in which the solution is heated to 230° C. ata pressure of 45 bar. From the pre-heater, the solution passes via anorifice into a helical-tube evaporator of length 15 m (internal diameter15 mm) in which the reaction solution reaches a temperature of 205° C.and a pressure of 7.8 bar downstream of the orifice. The reactionmixture is passed via a pipeline of length 6 m into a vessel.Simultaneously, 40 kg/h of 15% strength aqueous sodium hydroxide aremetered into this vessel. The resultant aqueous polyaspartic acidsolution has a hydrolysis number of 2.09 mmol of NaOH/g of solution.

Preparation of Solution of an Na Salt of Polyaspartic Acid

3000 g of solution of an Na/NH₄ salt of polyaspartic acid are prechargedand 501.6 g=6.27 mol of 50% strength NaOH solution are added. Thetemperature is increased to 100×110° C. and, with addition of 2×600 g ofwater, 3×600 g of ammonia-water are distilled off. The residue is 2901.6g of a 42.8% strength solution of an Na salt of PAA.

Molecular weight distribution ZnO dispersion test NACE-test by GPCM_(w)10-300 mg 3 ppm/10 ppm [g/mol] [% of theory] [% of theory] 1450 64 60/79

Example 3

The example described below was according to the general methoddescription b) (prior art carried out).

Preparation of Solution of an NH₄ Salt of Maleic Acid

An amount of 40 kg/h of a solution of an NH₄ salt of maleic acid,temperature-controlled at from 100 to 105° C. and prepared as in Example1, is pumped to a kneading apparatus for polycondensation.

Preparation of the Polycondensate Melt

The vessel is connected to the kneading apparatus via a pipeline oflength 29.4 m (internal diameter from 10 to 15 mm) which is heated to100-10° C. In the List (CRP 12 Konti) kneading apparatus the reactionmixture is polymerized at temperatures of from 190 to 195° C. androtation rates of 31/min. An amount of about 22 kg per hour is obtainedof the resultant polycondensate, a high-viscosity melt. It has ahydrolysis number of 10.51 mmol of NaOH/g of melt.

Preparation of Solution of an Na Salt of Polyaspartic Acid

2100 g of water and 360 g=9 mol of NaOH are precharged, and 1000 g ofpolycondensate melt are added little by little at 20° C., with stirring.During this the temperature rises to 60° C. through exothermicity andthe polycondensate dissolves. A further 60.4 g=1.51 mol of NaOH areadded, the temperature is increased to 100-110° C., and, with additionof 2×700 g of water, 3×700 g of ammonia-water are distilled off. Afteradding 175.6 g of water, 3000 g of a 39.1% strength by weight solutionof an Na salt of polyaspartic acid are obtained.

Molecular weight distribution ZnO dispersion test NACE test by GPCM_(w)10-300 mg 3 ppm/10 ppm [g/mol] [% of theory] [% of theory] 1450 34 42/69

What is claimed is:
 1. Method for performing polycondensation reactionsto form a polycondensation product, wherein, in a first reaction step, amonomeric starting material is produced by an exothermic reaction andthe temperature of the monomeric starting material is controlled viacooling before said monomeric starting material is used in a secondreaction step in which the polycondensation of the monomeric startingmaterial is performed with external supply of heat in a reactorcombination which has at least two stages and is composed of apre-reactor and a high-viscosity reactor, where low-molecular-weightelimination products produced are removed by evaporation, furtherwherein reaction product becomes concentrated in the pre-reactor to givea high-viscosity preliminary product and the preliminary product reactsto completion in the high-viscosity reactor with supply of thermal andmechanical energy and with a residence time of from 20 s to 60 min inthe high-viscosity reactor to give the polycondensation product. 2.Method according to claim 1, wherein the monomeric starting material isobtained by reacting maleic acid, fumaric acid, malic acid, asparticacid, maleic anhydride or mixtures thereof with ammonia or with acompound supplying ammonia.
 3. Method according to claim 1, wherein anaqueous solution of an ammonium salt of maleic acid is used as themonomeric starting material and polysuccinimide (PSI) is thepolycondensation product.
 4. Method according to claim 1, wherein thereaction in the pre-reactor is performed at a pressure of from 0.01 barto 100 bar, at temperatures above 100° C. and with a residence time offrom 0.5 min to 300 min, and the reaction in the high-viscosity reactoris performed at a temperature of from 100° C. to 300° C. and at apressure of from 0.01 bar to 10 bar.
 5. Method according to claim 4,wherein the reaction in the pre-reactor is performed at temperatures offrom 100° C. to 250° C., at a pressure of from 0.1 bar to 25 bar, andwith a residence time of from 1 min to 20 min, and the reaction in thehigh-viscosity reactor is performed at temperatures of from 120° C. to250° C., at a pressure of from 0.1 bar to 3 bar, and with a residencetime of from 20 s to 60 min.
 6. Method according to claim 3, wherein anaqueous solution of an ammonium salt of maleic acid with a molar ratioof nitrogen in the ammonium salt to the maleic acid of from 0.1 to 25,is used as the monomeric starting material, further wherein theproportion of water is from 20 to 90% by weight.
 7. Method according toclaim 3, wherein the aqueous solution of an ammonium salt of maleic acidis fed to a pre-reactor constructed in the form of a helical tube andthe polycondensation product in the high-viscosity reactor is agitatedand comminuted by rotating kneading elements and/or shearing elements.8. Method according to claim 7, wherein the polycondensation product inthe high-viscosity reactor is concentrated to give free-flowing solidparticles.
 9. Method according to claim 1, wherein the polycondensationproduct obtained in the second reaction step is subjected to solvolysis.10. Method according to claim 1, wherein the polycondensation producthas recurring aspartic acid units.
 11. A method of using thepolycondensation product obtained by the method of claim 1, wherein saidpolycondensation product is added to aqueous or nonaqueous systemscontaining inorganic or organic particles to disperse said inorganic ororganic particles.
 12. Method for performing polycondensation reactionsto form a polycondensation product, wherein, in a first reaction step, amonomeric starting material is produced by an exothermic reaction andthe temperature of the monomeric starting material is controlled viacooling before said monomeric starting material is used in a secondreaction step in which the polycondensation of the monomeric startingmaterial is performed with external supply of heat in a reactorcombination which has at least two stages and is composed of apre-reactor and a high-viscosity reactor, further wherein reactionproduct becomes concentrated in the pre-reactor to give a high-viscositypreliminary product and the preliminary product reacts to completion inthe high-viscosity reactor with supply of thermal and mechanical energyand with a residence time of from 20 s to 60 min in the high-viscosityreactor to give the polycondensation product which is discharged fromthe high-viscosity reactor along with low-molecular-weight eliminationproducts produced in the pre-reactor and the high-viscosity reactor. 13.Method according to claim 1, wherein the monomeric starting material isproduced in the first reaction step in the presence of a solvent. 14.Method according to claim 13, wherein at least a portion of said solventis removed in the pre-reactor by evaporation.
 15. Method according toclaim 13, wherein said solvent is water.
 16. Method according to claim1, wherein the temperature of the monomeric starting material in thefirst reaction step is controlled so as to be from 100 to 105° C. beforesaid monomeric starting material is used in the second reaction step.17. Method according to claim 9, wherein the polycondensation productobtained in the high-viscosity reactor is subjected to hydrolysis toform a hydrolyzed polycondensation product.
 18. Method according toclaim 1, wherein the hydrolyzed polycondensation product has recurringaspartic acid units.
 19. Method according to claim 1, wherein theviscosity of the high-viscosity preliminary product is greater than 200mPas.
 20. Method according to claim 1, wherein the viscosity of thehigh-viscosity preliminary product is greater than 500 mPas.